The present invention relates to an apparatus for generating a plurality of charged particle beamlets. Charged particle beams are used in a variety of systems, like lithography and microscopy systems. Some of these systems use a single source generating one beam which is subsequently split into a plurality of beamlets. The charged particle source used in these systems emits a charged particle beam with a defined opening angle, i.e. a diverging beam. The diverging beam often needs to be collimated, i.e. transformed into a parallel beam. In most applications a lens or lens assembly is used to refract the diverging beam emitted.
However, the change of angle of a charged particle beamlet is not exactly defined due to so-called chromatic aberrations. As a result the spot size of the beamlets on the target to be exposed or imaged also increases.
In GB2340991 and in U.S. Pat. Nos. 5,834,783, 5,905,267, 5,981,954, 6,124,599, 6,137,113, 6,166,387, 6,274,877, and the Journal of Vacuum Science and Technology B18 (6) pp. 3061–3066, charged particle beam lithography systems are disclosed, comprising a lens assembly for refraction of a diverging beam into a parallel beam. After refraction, the beam is split up into a plurality of beamlets using an aperture array. In these lithography systems an image of the apertures is projected on the surface to be exposed. To decrease the spot size, the image is reduced by a factor 200. The aberrations are dominated by the aberrations of the last lens in the system and not by the aberrations of the collimation assembly. However, these aberrations will affect the performance of the system.
The lens assembly in such systems has a chromatic aberration Δβ=C·ΔE/E·β, wherein β is the angle of an incoming ray with respect to the normal of the lens assembly and E the energy of the incoming charged particles. If Δβ is comparable to the intrinsic angle in the beam, it contributes to the size of the spots, which are formed on an object, e.g. a wafer with resist, to be processed.